>I do wonder though, how does it work when only one side practice it?
Take an extreme example: Ever witnessed someone calmly deescalate a very emotionally angry person? The person who deescalated knows tactics that work even when the other side doesn't practice it. Let's go to the other extreme: If I'm a bully, I can use verbal communication styles that work even if the other person doesn't practice it.
I've read a few communications books, including NVC. The overlap amongst them is large. I would also recommend negotiation books like Getting Past No - they are essentially books on communication with difficult people applied in a negotiation context.
All of them are about two things:
1. Making yourself a better communicator in expressing your perspective.
2. Drawing out others to express what they are thinking.
The second addresses your concern. There are approaches that make the other person feel safe (as opposed to defensive) in sharing his concerns. Once a person feels secure that you will not judge them, nor will you fight them, they will talk to you in a calmer manner. And therefore it helps to know what judgmental language sounds like and eliminate it from your rhetoric.
I once attended an NVC seminar series. It was not really worth it compared to the book, but one thing the instructor hammered into our heads was very useful: "Everything a person says or does is in service of his own needs". When you start observing the world with this in mind, it becomes fairly obvious. NVC puts heavy emphasis on this: Everything someone is saying (or not saying) is to serve some need of his. Your mission is to identify that need. If you've read negotiations books, you'll see the parallels.
So even if the other person is a horrible communicator - say he shouts and throws tantrum whenever unhappy - you'll realize that his behavior is not about you, but about him. It makes it much easier not to be put off by that behavior. If he is your boss and is muttering stuff about your incompetence, you'll know not to have identity issues about it. You'll learn to ignore all the insults, but not ignore the message. You'll learn how to engage with him to discover his need, and it usually pays off (nothing is 100% perfect).
The above paragraph is the ideal. I'm certainly not even near there. The instructor said it took about 5-7 years of trying to practice NVC before he reached that level. It's hard to change your own way of thinking and your own style of communication that you've developed over several decades. Don't expect quick results - changing your communication style is a long term plan.
I can, say, though that whenever I'm calm enough to use NVC skills, it usually works quite well. The conversational "templates" feel awkward an unnatural, but people will not notice them. What's more: It's very easy to recognize when others use such templates. And you'll notice that they've always spoken that way and you never noticed it - so it's not that unnatural. And you'll also notice others' successes in using them.
The NVC book is very prescriptive, and is light on, say, rigor (e.g. no studies cited, etc). But as I mentioned above, other books (some of which are from academics) usually have the same advice. In my experience the NVC book and Crucial Conversations book are the simplest to pick up and practice.
Now there is a whole NVC movement, which may be a little off-putting. It's become similar to Agile. Lots of chapters all over the world. Lots of focus on becoming a certified instructor. Lots of other baggage (spiritualism, socialism, activist movement). If you ignore all that and stick to it as a communication style, you'll be fine.